Question: Problem when substituting terms in an equation

I have the following equation:

eqn := m*`ℓ`^2*(diff(q[1](t), t, t))+k[t]*`ℓ`^2*q[1]+5*k[r]*q[1]-4*k[r]*q[2]+k[r]*q[3]-5*P*`ℓ`*q[1]+2*`ℓ`*P*q[2]+2*`ℓ`*T*q[1]-`ℓ`*T*q[2] = 0;

And I want to execute the following substitutions:

k[r]/(k[t]*`ℓ`^2) = alpha;
P/(k[t]*`ℓ`) = sigma
T/(k[t]*`ℓ`) = theta

For that, I have tried:

eqn1:= algsubs({k[r]/(k[t]*`ℓ`^2) = alpha, P/(k[t]*`ℓ`) = sigma, T/(k[t]*`ℓ`) = theta}, %);


eqn1:= algsubs(k[r]/(k[t]*`ℓ`^2) = alpha, %);
eqn2:= algsubs(P/(k[t]*`ℓ`) = sigma, %);
eqn3:= algsubs(T/(k[t]*`ℓ`) = theta, %);

But none of these have worked. Does anyone know how to solve this?

Thanks in advance.

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